GEOPHYSICAL SURVEY AT ABBATIJA TAD-DEJR HYPOGEUM (RABAT, MALTA)

In order to verify both the presence of hypogean structures and tombs, integrated geophysical surveys, with ground penetrating radar (GPR), and electrical resistivity tomography (ERT), were undertaken at ABBATIJA TAD-DEJR HYPOGEUM (RABAT, MALTA). Furthermore a three-dimensional model of the entire surveyed areas was created. The integration of the acquired data allows us to plan future excavations. The surveyed areas are showed in Fig. 1. The GPR data were collected along grids with parallel profiles set 0.5 m apart, using the Cobra georadar system with the 400MHz and 70MHz antennae. GPR data were processed in 3D using GPR-slice software (Goodman et al. 1994). In order to eliminate a small noise component and make it easy to interpret the GPR data, the following processing sequence was applied: 1) Zero-time adjust (static shift), in order to associate zero-time with zero-depth; 2) Frequency filtering, in order to remove high-frequency noise; 3) Migration, in order to correct the shape and dimension of reflection events related to the structure present in the subsoil. GPR data were visualized in 3D using both time slice and iso-surface amplitude (Conyers 2012). In this work, the time-slice technique has been used to display the amplitude variations within consecutive time windows of width ?t = 5 ns. Three-dimensional amplitude iso-surface rendering displays amplitudes of equal value in the GPR data volume. Shading is usually used to illuminate these surfaces, giving the appearance of real archaeological structures. In this case, the threshold calibration is a very delicate task in order to obtain useful results. In order to define the depth of archaeological remains the electromagnetic (EM) wave velocity, using the characteristic hyperbolic shape of a reflection from a point source (diffraction hyperbola), was used (Conyers 2012). ERT data were collected in a 3D mode along non-conventional profiles (Leucci et al. 2016) using Dipole-Dipole array (Loke 2001) and 1.5m electrode spacing. The ELECTRA, multichannel digital resistivimeter (moho srl) with 32 active channels was used for geoelectrical measurements. The ErtLab inversion software (http://www.geost udias tier.it/) was used for 3D total volume resistivity data distribution in the subsurface.